Method for filter fabrication

ABSTRACT

Method for the fabrication of a neat front side of a filter where the front side is inserted into an injection mold tool. Then a suitable elastic thermoplastic material is injected into the tool. The amount of elastic thermoplastic material injected is less than the available space so that the pressure will rapidly drop as the material flows from the point of injection. By injecting the elastic thermoplastic material into a gasket part this can be given an exact shape. At the same time the free front of the elastic thermoplastic material will secure that the pressure exerted on the filter matrix will not destroy this.

[0001] For efficient manufacturing of filters it is known to insert apleated or folded filter matrix into an injection molded plastic frame,glue is then filled into the space between the filter matrix and theframe to fix and seal the filter matrix in the frame. The glue is thenallowed to harden. This may take some time and either means a slowingdown of the manufacturing process or require cost increasingintermediate storage before the filter is packed or subjected toadditional working steps. The glue must have precisely the rightviscosity when administrated to the space between the filter matrix andthe filter frame. This means that without proper and costly apparatus agreat many filters have to be rejected because of leaks and they mayalso at times lack in appearance with glue here and there. There existtherefor at this type of filter a need for an improved way to seal thefilter matrix in the frame.

[0002] In accordance with the invention the above object is solved bythe following steps:

[0003] a) at the fabrication of the filter matrix the pleats areentirely sealed in the ends, this can be achieved with thicker orcontinuous glue strings for instance (or dipping the ends in glue);

[0004] b) then said filter matrix is placed in a frame

[0005] c) a tool is held against said filter and said frame and;

[0006] d) a heated sealing thermoplastic material is injected underpressure into the space between said filter matrix, frame and tool. Thevolume of the injected material is less than the actual volume of thespace between filter matrix and frame, so that the entire space is neverentirely filled.

[0007] The injected thermoplastic material will in the same process stepbe shaped by the tool and form a seal between filter matrix and frame.

[0008] Surprisingly enough the expected total collapse of theunsupported filter matrix does not occur. The reason for this ispresumably that although the pressure of the entering thermoplasticmaterial is very high the pressure at the propagation front of theinjected material will be approximately the same as the surroundingatmospheric pressure. From the injection point the pressure will thusfall rapidly, preventing the total pressure on the filter from crushingthe pleats, which would inevitably occur if a remaining free space wasnot left unfilled.

[0009] The high pressure used with injection molding technique makes itpossible with a fast filling process and since the space between frameand filter matrix is comparatively narrow the thermal content of thesealing material will rapidly be taken by the surrounding, frame, filtermaterial and air so that also the hardening will be fast. Since thefilter pleats are sealed in their ends it is not necessary for the sealbetween the matrix and the frame to cover the entire ends of the pleats,it is instead sufficient if only a continuous seal is established allaround the filter matrix. Since the seal material is only visible on thetop of the filter the local amount or how far deep the sealant extendsdoes not influence the appearance of the filter. The pressure of thesealant when injected is still sufficient to result in a good fillingout of the tool so that a very controlled appearance of the ready filtercan be achieved.

[0010] If too much material tend to agglomerate at some location thepropagation front will there advance down into the space preventing thebuild up of too high pressures and forces on the pleats. However sincethe space normally becomes more narrow downwards the flow resistancewill increase inducing an even distribution around the filter. The endsof the filter pleats will also function to this end since the pleatstaper downwards.

[0011] In particular at large filters it may become necessary withseveral sealant inlets into the space between the matrix and the framein order not to reach too high pressures locally. This is undesiredsince every inlet is comparatively expensive and there are manydifferent filters meaning high tooling costs. A remedy for this is in afurther development of the invention to use the filter frame itself orthe filter frame in combination with a molding tool or indeed some otherpart of the filter to obtain a distribution channel for the sealantaround the filter.

[0012] In order increase the pressure and thus the transport in thechannel without increasing the pressure and thus forces on the filtermatrix for instance holes in a wall or flange of the filter throughwhich the sealant has to flow can be used.

[0013] Alternatively a protective layer on the filter matrix that extendlaterally out from the matrix may have openings allowing only arestricted flow into the sealing area. The openings or passages may beof different size and distribution to promote an even distribution allaround the filter. This may be used to save tooling costs. Saidprotective layer may be a transport protection that is removed when thefilter is to be used or may be permanent and air permeable.

[0014] Normally a filter is then in a subsequent step completed by theapplication of a gasket. This gasket may be of foam that is applied ontop of a lateral flange or groove on this by a nozzle following thecontour of the filter. By careful control of viscosity nozzle size,speed etc the gasket can be given comparatively accurate dimensions.This way of fabricating the gasket is however due to the manyrequirements troublesome and frequently result in adjustment work andrejection which is most undesired. In accordance with a furtherdevelopment of the invention it is also possible to achieve this gasketin a simple, reliable and fast manner.

[0015] This is achieved by a common manufacturing step in which thefilter matrix is sealed relative the frame and the surrounding gasketfor the frame molded by the tool. An elastic thermoplastic material isused for the gasket and the seal between filter matrix and frame.

[0016] The channel in the tools or between tool giving the gasket canadvantageously be used also to distribute the sealing material aroundthe filter to give a more uniform distribution before the sealant entersinto the space between matrix and frame. Also possible holes in theframe become hidden between gasket and seal and at the same time theycan serv to “rivet” the gasket to the frame. By using holes or passagesfrom the gasket “channel” in the tool the elastic material is forced tofill out the gasket before entering into the seal space, which willtherefor occur comparatively uniform around the filter.

[0017] Additional advantages and characteristics of the invention areapparent form the following description of preferred embodiments of theinvention shown with reference to the enclosed drawings.

[0018] Here FIG. 1 shows a part cross section of a filter in accordancewith the invention and

[0019]FIG. 2 a part cross section of a second embodiment of a filter inaccordance with the invention.

[0020] The filter shown in FIG. 1 is constituted by a filter matrix 1and a filter frame 2. An integrated gasket 3 and a seal 4 between thefilter matrix 1 and the frame 2 is shaped by an upper tool half 9.Elastic thermoplastic material is injected through an inlet 4 into achannel 5 located on the upper side of a lateral flange 6 of the filterframe 3. The channel molds a gasket. The channel is in communicationwith the space 7 between the filter matrix 1 and the frame 2 by grooves8 in the upper tool half 9 that otherwise is in contact with the framehereby controlling the lateral position of this. The bottom tool 11 maybe the same tool that is used when previously injection molding thefilter frame 2.

[0021] The channel 5 is provided with a sawtooth cross section so thatthe achieved gasket will have a number of flanges or lips extendingupwards. The grooves 10 between the flanges give restrictions for thesealing material and thereby force this to flow along the channel togiving an even distribution along the sides of the frame. Bydimensioning these grooves with the corresponding flanges in the uppertool half as well as the channel on the upper side of the flange and theconnecting holes it is possible to achieve a far going distribution of asealing material around the filter already before this sealing materialstart to flow into the space between the frame and the filter matrix. Inthis way a uniform filling or injection of the sealing material betweenfilter matrix frame is achieved.

[0022] Instead of the grooves a narrow slot may exist between tool andframe for the restricted sealant flow providing the desired pressuredrop between the injection molded part and the filled out space.

[0023] The embodiment in FIG. 2 has the gasket 23 on the bottom side ofthe flange 26 and holes 28 in the side of the frame to let sealingmaterial into the space 27 between matrix 21 and frame 22.

[0024] Holes may also extend through the flange to shape a gasket on topof the flange and the sealant may be allowed also to enter into thespace between matrix and frame from the top as well as through the sidesof the frame.

[0025] The filter matrix is made of pleated filter material that beforethe final pleating has been provided with glue strings for gluingtogether the filter pleats to a filter matrix. The glue strings are butfor those along the edges intermittent in order to promote thetriangular shape of the filter pleats since this is considered to be thebest shape from flow considerations. The glue strings along the edges,perhaps some millimeter from the edges however include more materialsecuring that the filter matrix when the filter material is pleatedtogether will become entirely sealed between the filter pleats. Thismeans that it is only necessary with a seal around the upper (or lower)edge of the filter matrix in order to seal the matrix in the frame. Inthe narrow space between the filter matrix and the frame the ends of thepleats constitute a sick-sack shaped restriction that closes to theupper surface of the filter has a comparatively large cross sectionwhile the restriction has to be past with the reduction in cross sectiondownwards resulting in turn in a successively increased counter pressurefor the entering sealing material supporting the distribution of thesealing material around the filter. In the ends of the filter the lengthof the filter pleats are located and no similar restriction exist and itis here easier for the sealing material to flow down between the matrixand frame. Therefor the openings leading into this space are smaller toreduce the pressure. Also the filter matrix can here be made stronger byan increased number of additional glue strings, only extending over oneor a pair of pleats.

[0026] By means of the invention it becomes possible to achieve in onestep fixation and sealing of the filter matrix in the frame and themanufacture of the gasket that may be located on the top or bottom of aflange or indeed on the outside of the filter frame.

[0027] Since the gasket is shaped with tool it will receive exact andidentical look from filter to filter improving the look of thefabricated filter. The number of steps in the manufacturing process isreduced as is the cost.

1. Method for sealing of a filter matrix of pleated filter material in a filter frame, wherein: a) at the fabrication of the filter matrix the pleats are entirely sealed in the ends; b) then said filter matrix is placed in a frame c) a tool is held against said filter and said frame and; d) a sealing thermoplastic material is injected under pressure into the space between said filter matrix, frame and tool. The volume of said injected thermoplastic material is less than the actual volume of said space between said filter matrix and frame.
 2. Method according to claim 1, wherein the sealant is distributed around the filter frame by a channel in the tool facing the frame and giving a remaining corresponding ridge or bar on this and that the filter frame is provided with connections, or holes for a transfer of sealing material from the distributing channel to the sealing space between the matrix and frame.
 3. Method according to claim 2, wherein the holes are so arranged in the frame in relation to the flow path of the sealant that sealant arrives into the space between frame and matrix at different times, allowing the material that arrives first to harden slightly before the next arrives in order to give the second flow an additional flow resistance at a certain depth in the space.
 4. Method according to claim 1, wherein that the sealant is distributed around the filter frame by a channel in the tool facing the frame and giving a remaining corresponding ridge or bar on this and that between the filter frame and the tool is provided connecting slots, passages or openings for a transfer of sealing material from the distributing channel to the sealing space between the matrix and frame.
 6. Method according to claim 1 wherein the tools only hold around a flange or edge of the filter frame and cover the space between matrix and frame.
 7. Method according to claim 1 wherein thermoplastic material is elastic and the distribution channel is used to form a gasket for the filter.
 8. Method according to claim 1 wherein the part formed by the distribution channel is used to hold a protection for the filter in place until it is to be used.
 9. Filter frame for conducting of the method in accordance 2, which is provided with perforations holes or grooves for letting through sealing material from a bordering surrounding tool channel into the space between matrix and frame.
 10. Method for sealing of a filter matrix relative a gasket wherein: a) at the fabrication of the filter matrix the pleats are entirely sealed in the ends; b) a tool is held against the filter matrix and surrounds this at least partially and; c) a sealing thermoplastic material is injected under pressure into the space between the filter matrix, and the tool, the volume of the injected material being less than the actual volume of the space between filter matrix and tool.
 11. Method for sealing of a filter matrix of pleated filter material in a filter frame, wherein: a) at the fabrication of the filter matrix this is sealed all around; b) then said filter matrix is placed in a frame c) a tool is held against said filter and said frame and; d) a sealing thermoplastic material is injected under pressure into the space between said filter matrix, frame and tool. The volume of said injected thermoplastic material is less than the actual volume of said space between said filter matrix and frame. 